The present invention relates to a method for manufacturing a pneumatic tire including a tire rubber member of accurate sectional shape and with which it is possible to improve productivity, and to a pneumatic tire obtained thereby.
There is recently suggested a strip-wind method as illustrated in FIGS. 14 to 16. In the strip-wind method, there is formed a wound body in which a ribbon-like non-vulcanized rubber strip g is spirally wound around an object c to be wound. A green tire employing such a wound body is vulcanized and molded. In this manner, the wound body can comprise a tire rubber member such as a tread rubber tr. Such strip-wind method is suggested in, for instance, Japanese Patent No. 3,477,289, Japanese Patent No. 3,352,045, Japanese Patent No. 3,370,282 and Japanese Patent No. 3,397,430. The object c to be wound onto which the rubber strip g is wound can be a shrinkable mold former of cylindrical shape or a tread portion of a green tire in a condition in which a belt layer is wound around a green carcass.
In such a strip-wind method, a rubber member of arbitrary sectional shape can be formed by changing a winding pitch of the wound in the overlapped rubber strip. It will accordingly be possible to omit preparing extruding nozzle caps of specific sectional shapes for respective tires and exchanging operations of the caps, so that improvements in productivity can be expected, unlike a conventional integral extruding method.
FIGS. 13 to 16 illustrate some examples of sections of green tread rubber tr that are employed as conventional tire rubber members for forming a tread rubber. FIG. 13 illustrates an integral extruded article tr1 that has been integrally extruded from an extruder through a non-strip-wind method.
All of the tread rubbers as illustrated in FIGS. 14 to 16 have been manufactured by using the strip-wind method. The tread rubber tr2 of FIG. 14 is formed by spirally winding a single successive rubber strip g around a cylindrical object to be wound c in one direction from one side e1 to the other side e2. The tread rubber tr3 of FIG. 15 employs two separate rubber strips g1, g2. The one rubber strip g1 is spirally wound from one side e1 and the other rubber strip g2 from the other side e2 towards the center, respectively. Respective winding end points f1, f2 are located at substantially central portions in the width (width central portion). In FIGS. 14 and 15, the rubber strip g is illustrated in a schematic form with spaces being formed therebetween for ease of understanding.
The tread rubber tr4 of FIG. 16 employs two independent rubber strips g1, g2. The one rubber strip g1 is wound from a width central position of the tread rubber tr4 towards the one side e1 and the other rubber strip g2 from the width central position towards the other side e2, respectively.
The tread rubber tr thus obtained through a conventional strip-wind method is arranged, as illustrated in FIG. 17 in a plan view, in that the winding start point s and/or the winding end point f of the rubber strip g is/are located at sides e1, e2 of the wound body. It is accordingly the case that a stepped non-continuous portion k that comprises a corner when seen in a planar view is apt to occur between an end surface and an side of an adjoining rubber strip at the winding start point s and the winding end point f of the rubber strip g. Such non-continuous portions k lead to a drawback in that lateral sections of tread rubbers tr become non-uniform in the tire circumferential direction to thus worsen tire uniformity.
The winding start point s and the winding end point f of the rubber strip g are apt to peeling than other parts thereof. Accordingly, when those points are located at sides of the tread rubber etc, drawbacks are presented in that cracks or rubber chippings tend to occur from those portions after vulcanization.
On the other hand, there is also a case as illustrated in FIG. 18 in which the rubber strip g is spirally wound around by a single round in one direction from one side in the width direction to the other side similar to the case of FIG. 14. In such a wound body, the spiral winding pitch P of the rubber strip g will be extremely small for achieving a sufficient thickness. The rubber strip g is thus largely inclined obliquely in its sectional shape when being wound. A large peripheral velocity difference is generated between an outer end edge o and an inner end edge i of the rubber strip g in the radial direction. As a result, the rubber strip g tends to be pulled by the outer end edge o which peripheral velocity is large when being wound. It is difficult to wind the rubber strip g at specified positions in an accurate manner. Due to those facts, it is difficult to adjust the thickness (gauge) of respective parts of the tread rubber tr so that irregularities are apt to occur in the sectional shape in the above-mentioned method.
In the case of FIG. 18, the tread rubber tr is arranged in that both the winding start point s and the end point f of the rubber strip g are positioned at end portions of the tread rubber tr in the tire axial direction. The gauge at the end portions of the tread rubber tr will thus not become uniform in the tire circumferential direction. Since the tread rubber tr has a sectional shape in which its thickness gradually decreases in approaching outward of the tire axial direction, irregularities in gauge at end portions of the tread rubber will largely affect uniformity and other factors.
Referring to a concrete example, where portions of a thickness of 3 mm and portions of a thickness of 2 mm are formed at end portions of a tread rubber tr which are of small thickness, the irregularities of this gauge will amount to 33%. However, where portions of a thickness of 10 mm and portions of a thickness of 9 mm are formed at a central portion of the tread rubber tr which is of large thickness, the range of irregularity will be 10% so that influences on the uniformity will be relatively small.
Non-uniformity of gauges at end portions of the tread rubber tr (end portions of the wound body) is not desirable since they conspicuously worsen the uniformity of the tire and degrade the durability of the tire.